ci: correct paper-path key and add debug ls

Author: Jshulgach

.github/workflows/paper.yml

@@ -23,7 +23,9 @@ jobs:
         uses: openjournals/openjournals-draft-action@master
         with:
           journal: joss
-          paper_path: paper.md
+          paper-path: paper.md
+      - name: List files (Debug)
+        run: ls -R
       - name: Upload PDF
         uses: actions/upload-artifact@v4
         with: